Silencer having contiguous concentric layers of sound absorbent material



May 4, 1965 EDWARDS OF SOUND ABSORBENT MATERIAL Filed April 15. 1963 o c o o no a Q a A OOOODlOOUOUU a v Ir 2 FIG.|

FIG. 2

INVENTOR.

Howard C. Edwards ATTORNEYS United States Patent SILENCER HAVING CONTIGUOUS CONCENTRIC LAYERS OF SOUND ABSORBENT MATERIAL Howard C. Edwards, 2443 Whipple Road NW., Canton, Ohio Filed Apr. 15, 1963, Ser. No. 272,915 2 Claims. (Cl. 181-42) This application is a continuation-in-part of application Serial No. 228,194, now Patent No. 3,165,257, filed October 3, 1962.

This invention relates to the art of sound attenuation. More particularly the invention comprises the construction of silencers which are permeable, yet non-conductive, to the full spectra of noise, and which are especially efficient in the use of space and materials for dampening sound waves.

The art of mutfiing sound is highly developed. However, complete attenuation of greatly varied frequencies of noise normally requires a great amount of materials and consequently, a large space. Economical use of high speed equipment in working areas requires effective sound attenuation, using a minimum of materials. This is particularly true when new high energy sound producing devices, such as high speed air handling equipment are employed.

One object of the present invention is to provide a compact silencer which will absorb high energy noise, having great variety of frequencies. Another object is to provide a silencer which, while at the same time penetrable to all frequencies of sound and dead to their transmittal, will occupy minimum space. A further object of this invention is to apply a silencer, meeting the above limitations, to a venturi in such a manner that the greatest amount of sound attentuation will be realized with the least amount of materials.

Referring to the drawings:

FIGURE 1 is an exterior view of the silencer as applied ot the venturi of a pressure inducer;

FIGURE 2 is a sectional view of the silencer taken along line 3-3 of FIGURE 1;

The silencer is generally indicated in FIGURE 1 by the numeral 200. As may be seen in FIGURE 2, the silencer has as internal shell 212, which is perforated by apertures 214. Shell 212 is surrounded by a relatively tightly compacted layer 216 of sound absorbent material, which may be asbestos or glass wool. Wrapped around and directly contacting layer 216 is a relatively loosely compressed layer 218 of a similar sound absorbent material. Laminated shell 220 bounds layer 213. Rigid, usually metallic, laminates 222 and 226 are separated by a non-rigid, porous layer 224, which may be made of asbestos. Apertures 228 penetrate the exterior shell 220 at intervals calculated to misalign the apertures with perforation 214 of the internal shell. Because of this misalignment, a slight pressure of air is developed in apertures 214 of the internal shell, relative to the air pressure existing in apertures 228. The air pressure at apertures 214 help to prevent the material which is being comducted through the silencer from hitting the sides thereof.

As indicated in application Serial No. 228,194, now Patent No. 3,165,257, the respective elements of the silencer including internal shell 212, sound absorbent layers 216, 218 and 224, and rigid laminates 222 and 226 3,181,646 Patented May 4, 1965 are tapered with respect to a central axis. The taper defines a conical tubular section, as shown.

The construction of the silencer allows free penetration of sound waves from their source into the silencer, while denying further passage of the sound waves out of the silencer. The internal structure of the silencer is designed to provide maximum interference to the waves. The tightly compacted material 216 best absorbs one range of frequencies while the loosely compacted material 218 absorbs another. Moreover, both layers 216 and 218 cooperate to attenuate sound waves passing between them. Waves that pass through one layer find the next layer less penetrable to their transmission, but not so abruptly impervious as to echo waves into the first material. Thus, it is ditficult to produce resonance or standing waves in the silencer.

Inner laminate 222 is a rigid surface against which sound waves echo back into the mufliing material. Vibrations imposed upon laminate 222 by impinging waves are damped by the intermediate sound absorbent laminate 224, and, consequently are not passed to the outer laminate 226. Laminate 224 is especially elfective against high frequency sound waves which penetrate layer 216 and 218.

FIGURE 1 shows the silencer in its particular application to the venturi of a high pressure draft inducer, which is generally indicated by numeral 100. High pressure air is pumped by fan through conduit 112 into plenum 114. Pressurized air spills from plenum 114 into venturi 116 in a direction parallel to its axis. The induced draft passes through plenum 114 in an enclosed pipe which opens into the constricted portion of venturi 116. Noise is produced by lateral vibrations, resulting from mixing air currents under greatly varied pressures. Since a wide range of vibrations is created, resultant noises vary greatly in intensity and frequency. Outstanding, however, are high frequency noises.

The apex of induced draft in the venturi occurs at a point adjacent to the maximum inner circumference of the tube. At this point the noise produced is loudest, and it is this point at which the greatest surface of sound insulating material contacts the venturi. In practice, the silencer is extended beyond the end of the venturi by section 230 shown in FIGURE 1. While passing through the section of the duct covered by silencer extension 23%), the entire gaseous flow becomes parallel to the axis of the pipe. Thus, beyond this first portion of the duct vibrations, noise and the accompanying need for sound attenuation are minimal.

Although the silencer has been described by Way of a specific example in which the elements form concentric tubes, it may be seen that particular silencer construction would be useful in any form to isolate one area from noise produced in another. Necessarily, when extremely high energy noise is presented, the silencer must be constructed of suflicient thickness to attentuate the sounds. Usually simple bulk is added; This invention, however, teaches the addition of adjacent layers of sound absorbent material having varied compressions. Interaction of the layers offers more efiicient use of space and materials.

I claim:

1. A silencer comprising the following elements in contiguous relationship: an outer shell comprising a first layer of porous sound absorbent material bounded by two rigid laminates, said outer shell being perforated with apertures; a second layer of loosely compressed sound absorbent material; a third layer of tightly compressed sound absorbent material; and an inner shell perforated with apertures, the apertures in said inner and outer shells being located so as to be misaligned with respect to each other.

2. The silencer of claim 1 wherein the respective elements are tapered with respect to a center axis.

References Qited by the Examiner UNITED STATES PATENTS Lambert 181-362 Faust 181-60 X Engels 181-42 Lemrnerman 181-50 X Baruch 181-50 Rawson 181-42 Buttler 181-62 X Addie et al 230-233 X Sanders 230-233 FOREIGN PATENTS Great Britain.

LEO SMILOW, Primary Examiner. 

1. A SILENCER COMPRISING THE FOLLOWING ELEMENTS IN CONTIGUOUS RELATIONSHIP: AN OUTER SHELL COMPRISING A FIRST LAYER OF POROUS SOUND ABSORBENT MATERIAL BOUNDED BY TWO RIGID LAMINATES, SAID OUTER SHELL BEING PERFORATED WITH APERTURES; A SECOND LAYER OF LOOSELY COMPRESSED SOUND ABSORBENT MATERIAL; A THIRD LAYER OF TIGHTLY COMPRESSED SOUND ABSORBENT MATERIAL; AND AN INNER SHELL PERFORATED 